In a nuclear reactor power plant, a nuclear reactor vessel is used to generate heat for the production of steam and electricity. The reactor vessel is typically a pressurized vessel enclosing a core of nuclear fuel and cooling water. Such nuclear power plants typically contain three major components: a reactor vessel containing fuel which produces superheated water for transport to one or more steam generators, which output steam to drive a multi-stage steam turbine to generate electric power.
The superheated water is transported to the steam generator by pipes. These pipes feed the water into numerous tubes within the steam generator. These tubes are U-shaped, feeding the water back to the pipes at the outlet of the steam generator to be re-circulated back to the reactor. The tubes in a nuclear steam generator typically form an inverted “U” separated by a lane, and held together by a plurality of support plates, separated at periodic vertical intervals. The height of each tube row may exceed thirty-two feet. Six to eight or more support plates are used, each separated vertically at three to six foot intervals. In the steam generator, the tube carrying the superheated water are quenched with cool water, which generates the steam which drives the turbine to produce electricity.
This procedure for generating steam presents several problems. The water used to quench the tubes often has impurities and chemicals which may corrode both the steam generator tubes and the support plates and lead to other damage. Even though periodic inspections of nuclear steam generators are required for compliance with safety regulations, monitoring steam generator cleanliness remains a problem. The highly corrosive environment of the steam generator is particularly problematic for many of the older nuclear reactors in service throughout the world.
In the past, steam generator tubes and support plates were inaccessible for visual inspection. Information was gathered by complicated systems which could not adequately inspect all sections of tubes and support plates. Because of the highly radioactive environment and the heat of the pipes, direct visual human inspection has typically been restricted to between three and five minutes per man per six month period. This time period does not provide ample opportunity for the careful inspection for corrosion, holes and leaks. It is therefore difficult to inspect within the narrow lanes and tube separation gaps at the support plates, because of the heat, radioactivity and narrowness of the lanes separating the tubes.
Tubes typically extend through support plates at quatrefoil holes. These openings provide flow through features to improve water flow in the generator and to reduce the build-up of sediment at the support plates. Nevertheless, the small areas where the quatrefoil opening must contact the tube results in areas of material build-up on the tubes, or even adherence of material being “plated out” on the tubes. This material will contribute to premature corrosion of the tubes. With known inspection devices, this condition will go undetected on all but the tubes bordering the lane.
Further, the orientation of component parts within steam generators provides extreme challenges to designing workable devices for inspecting such areas. Insertion holes (also known as hand holes) at the bottom of the steam generators are often as small as a five or six inch diameter. For the purpose of this application such portals will be referred to inclusively as access ports. Flow distribution baffles within the generator often obstruct any room to maneuver equipment within the generator. Inspection within steam generators at elevations as high as thirty feet or more provide significant engineering challenges. In addition, the flow slots between tube rows are often less than two inches wide and tube separation gap dimensions are often less than one inch (down to about 0.30 inches).